The present invention relates to apparatus for securing a heat sink to electronic device package such as central processing unit for dissipation of heat therefrom, and more particularly to heat sink arrangement for central processing unit (CPU) which firmly holds the heat sink and evenly urges it into engagement with the CPU in intimate thermal contact with the maximum thermal contact area.
The most common way to improve the package thermal performance is to increase the surface area of the device by attaching a large piece of metal, a heat sink, to the package. The heat sink is usually made of Aluminum or Copper and is chosen for its price and thermal performance ratio.
Finned heat sinks have been found to be particularly effective when placed in contact with an electronic device package, such as a central processing unit (CPU), a microprocessor or a gate array, for transferring heat through conduction from the CPU and rapidly dissipating such heat to the environment by the enlarged surface area provided by the fins. Various heat sink assemblies are known in the art to thermally couple heat sinks with such CPU so as to dissipate heat generated by the CPU and thus lower the temperature of the CPU to safe limits.
It is known to use gripping assembly to hold a heat sink to a CPU, examples of such heat sink gripping assembly are shown and described in U.S. Pat. Nos. 4,745,456; 5,251,101; 5,423,375; 5,409,352; and 5,353,863. Such gripping structure is difficult to hold the heat sink in intimate thermal contact with the CPU due to uneven clamping force. In fact, clearance may easily be formed between the heat sink and the CPU which may significantly reduce the thermal coupling. Such gripping structure, while effective when the clamping force biases the heat sink into firm engagement with the CPU, such structure tends to be inconsistent as the CPU heats up and, at elevated temperatures, the force biasing the heat sink against the CPU can decrease making the thermal path less efficient.
Another known example of heat sink assembly is disclosed in U.S. Pat. No. 5,313,099. This patent provides a heat sink assembly adapted for removing heat from an electronic device package comprises an adaptor and a heat sink. The adaptor has a top wall and a pair of opposed side walls depending downwardly therefrom, in which a groove formed in each side wall extending generally parallel to the top wall in alignment with and facing each other and being capable of slidably receiving said electronic device package. The adaptor further has a first and a second planes lying respectively in the top and bottom of the grooves defining a space therebetween. The groove is adapted to dampingly receive opposite marginal portions of an electronic device package when inserted therein. The top wall has a bore centrally located and extended therethrough. The heat sink has a cylindrical base member with a fiat bottom surface adapted to be received through the bore in the top wall. The base member formed with means to engage with the periphery thread of the bore to secure and to attach the heat sink member to the adaptor with the fiat bottom surface disposed in the space between the first and second planes whereby an electronic device package slidably inserted in the grooves will cause the top wall to bow through engagement with the flat bottom surface of the base.
In accordance with a preferred first embodiment of U.S. Pat. No. 5,313,099, the bore is a threaded bore and the base member of the heat sink is provided with outer thread. The heat sink is screwed to the threaded bore and down toward the electronic device package until the flat bottom surface of the heat sink base member is firmly biased against the top of the electronic device package and thermally coupled thereto. According to another preferred embodiment, the base member is provided with an outwardly extending flange at its lower distal end and is provided with a tapered surface portion. The flange is adapted to snap through the bore in the top wall to lock the heat sink to the adaptor.
The flange structure of the base member according to the second embodiment has the following disadvantages. If the urging force applied to the top surface of the electronic device package by the base member is insufficient where the thickness of the base member is thinner than the thickness of the bore, it is found that the thermal coupling can be significantly reduced and thus increase the thermal resistance therebetween. If the urging force applied to the top surface of the electrode device package by the base member where the base member's thickness is larger than the bore's thickness, the overloaded urging force may damage the electrode device package. Hence the flange structure of the heat sink base member mentioned above is messy, tedious, labor intensive and therefore expensive and it is difficult to accurately manufacture various parts with desired dimension.
The screwing engagement structure between the base member and the bore according to the first embodiment simplifies the coupling operation of the heat sink assembly but it bears some unexpected adverse effects. Since the threaded bore is formed on the top wall of the adaptor and the electrode device package is received between the first and second plane, the surface area of the bottom surface of the base member which is screwed to the threaded bore should be smaller than the surface area of the top surface of the electrode device package. Generally, the surface area of the top surface of the electrode device package, for example a Pentium Processor, is 25 sq. cm, i.e. 5 cm.times.5 cm, wherein the top surface area of its major heat generating central portion, integral circuit portion, is approximately 3.5 cm.times.3.5 cm, i.e. 12.25 sq.cm.
However, due to the limited space provided around the electronic device package for installment, the maximum exterior size of the adaptor is 5.3 cm.times.5.3 cm and thus the adaptable diameter of the base member of the heat sink is 2.7 cm. So that the bottom surface area of the base member is 5.72 sq. cm, i.e. 1.35 cm.times.1.35 cm.times..pi.. Accordingly, only a limited small central area of the top surface of the electrode device package is in thermal contact with the bottom surface of the heat sink base member. In fact, only 46.69% of the major heat generating central portion of the top surface or 22.89% of the entire top surface of the Pentium Processor is in thermal contact with the heat sink.
It is well known that to maximize the flow of heat for a given junction temperature rise over the ambient temperature, the thermal resistance from heat sink to air can be reduced by maximizing the surface area, and maximizing the airflow across the surface area. Therefore, the larger thermal contact area may result the better thermal conduction. Hence the best thermal conducting condition of the heat sink is at least entirely in thermal contact with the major heat generating central portion of the top surface of the Pentium Processor. Conversely, the conventional structure of directly screwing the heat sink via an adapter upon the CPU limits the thermal contact area merely to the a limited small central portion of the top surface of the CPU with the result that it reduces the heat dissipating effect of the heat sink.
Moreover, when the operator screws the base member of the conventional heat sink as mentioned above into the threaded bore and down toward the electronic device package until the flat bottom surface of the heat sink base member is firefly biased against the top of the electronic device package, a downward urging force is applied to the central contacting portion of the electronic device package. Since the outer marginal portions of the housing of electronic device package is gripped between the lips and the top wall of the adaptor, the outer marginal portions of the housing of electronic device package will apply an upward reaction force to the top wall. Thus a bending torque force is formed to the electronic device package which may cause deleterious operational effects and damage the package.
On the other hand, the electronic device package is held in position by supporting its side edges with the side lips of the adaptor. During operation, although the bottom surface of the heat sink base member is already in contact with the horizontally supported electronic device package, the operator is still able to continue the downward screwing of the heat sink by bending the package that may cause adverse effects to the package. Besides, such over screwing of the heat sink upon the electrode device package may cause the bottom surface of the base member scratching upon the top surface of the electronic device package, causing unexpected adverse effects.